How do you diagnose and repair a bad 4-way switch?
Four-way switch. Far less common than the 3-way switch, the 4-way switch is likely to be found near stairs or along hallways. It is always installed in the circuit between a pair of 3-way switches. The combination of 3-way and 4-way switches allows someone to operate a fixture from three or more locations. For example, if you have three or more stories home, you can turn the stairwell lights on and off from every floor. A 4-way switch has 4 brass screw terminals and similarly to the 3-way switch it doesn’t have ON and Off markings on the toggle.
Free the switch from the box. Turn off the power to the switches at the service panel. Remove the cover plates of all 4-way switches that operate the light fixture. Identify the 3-way and 4-way switches: 3-way switches have three terminals and 4-way have four terminals. Begin by servicing the 3-way switches; if none of them is faulty, test the 4-way switches one by one. Loosen the screws on the mounting strap and pull the switch out of the box to expose the wires. There are two cables entering the box; all four wires connected to the switch are hot. Two wires connected to one side of the switch are red; two attached to the other side, are either black, or white recoded black.
Test for voltage and check the connections. Use a voltage tester to confirm that the power is off by touching one of the tester probes to the grounded metal box and the other to one of the four screw terminals. For plastic boxes, touch one probe to the switch terminal and the other to the green grounding terminal. Repeat the procedure for all four terminals. Next, check for loose or dirty connections. If necessary, clean the wires and terminals stripping the wires if necessary. Tighten the terminal screws and wire caps. Screw the switch back into the box and turn on the power. If the switch works. put on the cover plate. If it doesn’t, turn off the power again and take the switch out of the box to continue to the next step.
Test the switch. To help yourself when reconnecting the switch, use masking tape to tag the wires and disconnect the switch. Set your tester to ‘continuity,’ touch one of the probes to one of the upper terminals, and touch the other probe to the lower terminal (it should show continuity). If it doesn’t, toggle the switch to the other position and check it again. Next, touch the tester probe to the lower terminal on the opposite side and move the toggle to the other position. The tester should show continuity. Touch the opposite upper terminal with one of the probes and repeat all the above procedures. The tester should not show continuity. If the switch fails any of these tests, replace it. If the switch is good, reconnect it, install it back in the box, install the wall plate, and check any other four-way switches in the same circuit. If replacement is needed, look at the tag you previously put on the wires to connect them to the new switch exactly the same way they were on the old one.